Missing Players in an Ongoing Campaign

I've been running two ongoing campaigns the last few years, both more narrative than sandbox. What happens when a player can't make it? We deal with it and the game goes on. But how do you deal with it? OK let's discuss some details. Here are three rules I go by:

1. Never end a session in the middle of a fight
2. Set a minimum number of players who need to show to run your game
3. There's always a plausible explanation why Character X isn't here right now

Another wave?

Reasoning for Rule 1:
(This is really more about avoiding the awkwardness of having a missing player next time)

Say you have a 5-person party in the middle of a huge fight = "alright, it's two in the morning, we're going to have to call it til next time." Sounds reasonable, except that someone won't be able to make it next time - better hope it's not the cleric! Maybe two someones won't. Then another week goes by and someone else has a conflict. Odds are you're going to end up finishing the fight either a man down (or more) or so much time will pass that no one can remember the details from the first part of the fight and spell or power usage, hit points, and maybe even consciousness state are all very fuzzy. It dilutes whatever dramatic impact your fight might have and for me anyway is just an unsatisfying way to handle things.

Most people who give this advice don't do it because its's a nice theory - we do so because we've tried it and found how many ways it can go wrong. After a few years go by you might be tempted to try it again - nope, it still sucks.

Solutions:

• Never start a combat 5 minutes before "quitting time". My weekend night games generally run til midnight, but we've called it as early as 11:35 when it felt right.
• If you know they're starting a good-sized fight and you could freeze-frame before it starts but are tempted to push on, tell your players the situation and let them have some input. They might be cool running over a bit or they may already be running on fumes. 
• If you find yourself in the nightmare of fight-started-with-plenty-of-time-but-now-taking-way-longer-than-expected-and-it-needs-to-stop here are some emergency ideas:
• Bad Guys all beam/teleport/vanish out
• Bad Guys retreat through a hidden secret door to some new room you may need to map before next time. Darkness spells and smoke grenades can help here.
• Bad Guys heads explode and all fall dead - could be real, could be an illusion
• Bad Guys surrender - players never really expect this in the middle of a fight. If you can tie it to the death of a particular bad guy leader or a nasty move by one of the characters, so much the better.

These are much more satisfying without built-in excuses like "but the cleric wasn't here"

Reasoning for Rule 2:

Maybe your sessions are "game night" and you play something regardless of who and how many show up. That's cool and I envy you. Here it's "Campaign X Night" and we either play that or we don't play at all. The concept of the "backup game" has never really taken root with us. That said we very rarely cancel at the last minute, so setting that minimum number lets us figure out in advance whether we're going to run next week or not.

Examples

• I was running Red Hand of Doom in 4E with a party that varied from 5-6 players. If at least 4 can make it we play, if 3 or fewer we don't. It does mean you miss some sessions but it also means you don't have a TPK destroy your campaign because the groups was a man or two short the night they ran into a really nasty dragon. 
• In Wrath of the Righteous I knew I was going to run it mainly for two of my players, so they each made two characters (it's Pathfinder, so it's easy enough). Adventure Paths are written for 4 characters - presto, we're solid. If both of my two can't make it, we don't play that weekend. If someone else can make it on a day we do play, they can make up a new character or continue with the one they had created last time. If someone else starts showing consistently they might get to make a second character if they so desire. It's easier to adjust upwards on the fly than downwards, at least for me. 
• The Exception To This Rule: Superhero games - I've run sessions with only a single player running a single hero and still had plenty of fun. 

Spock's player couldn't make it that night - look at how that worked out

Reasoning for Rule 3:

If you've seen The Gamers then you've seen "Mark". Mark just appears in the background of the scenes, motionless and doing nothing. Then his player shows up and the character goes into action for one fight.  The player then has to go and his character pretty much disappears from the rest of the story. It's dumb but very true to life. 

Speaking from my personal view as a DM, I'm not here to run your character - that's your job and I have enough to manage. So I'm not going to run your character, Spock's Brain-style, in the background just because you're not here. My rule is pretty much "if you're not here then your character's not here". That pretty much eliminates the issue of characters dying in a session the player didn't attend, treasure shenanigans, and "I wouldn't do that!" conversations. 

Practical Considerations for Rule 3

• It's incredibly easy to explain character comings and goings in superhero games - they were "called away" or Lois is in trouble again or they had to go take some pictures for their day job. it's trivial and should never be a real problem. Unless you're running Time of Crisis, then it's a little tricky. 
• Wilderness adventures are great for this kind of thing - communing with nature, following some interesting tracks, leading a hostile monster away from the party, gathering some rare herbs, celebrating a high holy day in private, heading back to watch the road, spending quality time with his new dryad friend, acquiring a new familiar - these are all pretty easy to do. 
• Cities make this even easier - it's not hard to figure something out. Shopping, stealing, or carousing are all popular options.
• In contrast dungeons can make this really painful, especially the big ones. When the party is six levels deep, the DM still checks for encounters even on "cleared" levels, and there's no shortcut back out (Hello Town Portal!) then it can really strain belief to come up with a plausible reason why the fighter suddenly isn't going to fight for awhile. My advice is to think about this beforehand and try to come up with some possible explanations. Some ideas:
• Fell through a trap door into another room or another level - hey megadungeons are supposed to be "living" right?
• Taken prisoner by some group on this level or the next one
• Ran into a rival adventuring party and didn't want to lead them back to the group until he checked them out
• Wizards are studying in that last library/laboratory/summoning room the group discovered
• Clerics are purifying a defiled temple, defiling an enemy temple, or communing with their god in some other room
• Thieves are off sneaking around looking for more loot - you know how they are. A good candidate for "taken prisoner" above. 

Sure, let the DM run your character while you're gone ...

Final Thought: If you're really stuck for ideas ask the player what their character is doing while they are out. Their third or fourth idea is probably good enough to use.